Download Seafloor Morphology - Department of Geology UPRM

Seafloor Morphology
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f we select a grid for the surface of the earth (i.e. 5
km2) and assign it an average elevation in relation to
sea level, we can construct a graph of elevation versus
area of the Earth’s surface. This hypsographic curve
shows the surface morphology of the planet.
The shape of the seafloor is dominated by tectonic
processes with igneous activity having a strong role.
Initial structural and volcanic creations are modified by
deposition of sediments. Erosional processes also modify
the seafloor, but are not as effective as they are on
shaping aerially exposed land surfaces.
A variety of tools are used to investigate the seafloor,
ranging from diving and submersibles to seismic,
magnetic, gravity and satellite observations.
The features are described and grouped into geomorphic
categories. Most are an expression of structural and
igneous activity.
Techniques of Investigation
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n this section, we will examine the methods used to obtain data about the topography (bathymetry) of the sea floor and
the sediment cover.
Bathymetry and Sediment Studies
O
ur earliest records of the ocean floor were soundings taken with a
lead and line. To plumb the depths of the ocean, lengths of piano
wire were used to lower a weight to the bottom. The Challenger
expedition ran from 1872 to 1876 and took 492 deep sea soundings.
Modern oceanography in the United States began with the
establishment of Scripps Institution of Oceanography as a biological
research station in 1903, and Woods Hole Oceanographic Institution in
1930. The major cruise of this period was the voyage of the German
ship, the Meteor, in 1925. This cruise carried two acoustic depth
recorders which made soundings from a stationary position every two
to three nautical miles, giving new detail on the configuration of the sea
floor. With the military expansion of practical oceanography in World
War II, studies of transmission of sound in the ocean resulted in
extensive use of fathometers in continuous-line bathymetric surveys of
the ocean.
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The fathometer is a fairly simple device consisting of a sender or sound source and a receiver for detection of the return
signal. The difference in density between the water and the sea floor sends a reflected signal back to the unit. The travel
time is directly proportional to the water depth
(and density of the seawater column). As the ship
moves, a continuous profile of the bottom is
recorded.
Side scan sonar towed behind a ship scans the sea
floor with two divergent sonar beams that
acoustically illuminate the sea floor in two
swaths, from a few meters to as much as 30 km to
each side depending on depth above the bottom of
the tow. The sonographs show local topography
as sonar shadows. This is used with a fathometer
to expand the bottom bathymetric coverage.
Actual depth is not measured. Reflectance as
measured by side scan sonar can be related to
sediment types so the nature of the bottom
sediments can be described.
Remote sensing from satellites has become an important tool in
Oceanography. Instruments on satellites are used to measure sea
temperature, ice cover, bottom sediments in shallow water, and
bathymetry. In shallow water, measurements are based on penetration,
and in deep water, the sea surface elevation can be measured by satellite
instrumentation reflecting larger features on the bottom. Satellite images
can also be equated to bottom types by reflectance values in water depths
less than 10 meters.
One of the earliest devices, and still important, are grab samplers for
bottom sediment collection. These samplers are lowered from the ship
until they reach bottom and collect a sediment sample. Where the bottom
is hard, a dredge of heavy metal and chain mesh is used to dislodge and
collect rocks.
Grab samplers
collect a surface
sample which
depicts only recent sedimentation patterns. Geologists have used
coring devices for many years to obtain a record of sedimentation
over longer periods of time. These are essentially tubes that are
pushed into the sediment layers to collect a complete layered
sequence of sediments and they are effective in unconsolidated
sediments to depths of more than 20 m.
For recovery of rock bottom of much greater lengths, cores are
taken with a shipboard rotary drill similar to equipment used in
drilling for oil.
Suspended sediments are taken from the surface with a bucket or
from different depths in the water column with water samplers. The
water can be filtered and the amount of suspended sediment
determined by weighing the residue. Light transmission is also used to determine the amount of sediment in the water
column. Longer term studies involve the use of sediment traps which are placed on or near the bottom to collect sediment
settling out of suspension. These are collected periodically and the sediment is recovered to determine deposition rates.
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Seafloor Features
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he three major ocean basins are the Atlantic, Indian, and Pacific oceans. These lie over oceanic crust and have an
average depth of about 3800 meters. The Atlantic Ocean is the youngest of the three and is dominated by a central
oceanic ridge and abyssal plains of fine sediment. The Atlantic Ocean has grown during the past 200 million years at the
expense of the Pacific Ocean.
The oceanic provinces range from shallow coastal areas to the deepest ocean environments. Many of the ocean features
have been named and the particular provinces described. These features are discussed in the following sections.
Ocean Ridges
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he oceanic ridge system is a pronounced tectonic feature. The combined ridges are more than 60,000 km in length,
with an area of 23% of the earth's surface, almost equal to the total area of the continents. These ridges extend as an
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almost continuous feature around the globe in the form of spectacular mountain ranges of volcanic basalts. The ridges are
arched up and broken by numerous fault blocks to form linear hills and valleys. A prominent rift valley marks the crest of
the ridge throughout most of its length.
The general character of the ridge is a function of the rate of plate separation. A slow rate of spreading produces a higher
and more rugged oceanic ridge than when the spreading rates are more rapid. Rift valleys are also more prominent on
ridges between slow moving plates.
The rift valley along the center of the rise is a
zone of shallow earthquakes. The rift and
ridge system is not restricted to the oceans -it emerges in continental areas in Africa,
California, and in Iceland. Numerous open
fissures have been observed and mapped in
the rift valleys, which is evidence that the
crust is being pulled apart along the ridge. The
eruption of lava from these fractures parallels
the rift valley and creates long narrow ridges.
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The oceanic ridge is cut by faults normal to the ridge. Although these are strike-slip faults, vertical displacement may
form abrupt cliffs that can be traced for many kilometers . Horizontal movement on these transform faults is on the order
of 1-2 cm/yr and the faults are marked by earthquake activity and vulcanism. The magnitude of the system and its nature
indicates that it is related to major events and sources of energy in the Earth's interior.
Abyssal Hills and Abyssal Plains
T
he abyssal hills have relatively low relief as they rise only 75 to 900 meters above the ocean floor. Abyssal hills were
formed as an oceanic ridge. As the crust moved away from the spreading center, it cooled and sank to a lower depth.
The mountainous terrain of the oceanic ridge is maintained, becoming low-lying abyssal hills at depths of more than 6,000
meters. The hills are usually covered with a blanket of unconsolidated pelagic sediments deposited with reasonable
uniformity which gradually modifies and smoothes the features but does not change the original volcanic ocean floor
topography that formed at the ridge.
A flat featureless surface known as an
abyssal plain occurs when the hilly sea
floor has been covered by a thick fill of
sediments, which were deposited by
turbidity currents. These river-like
flows of a sediment-water mix are
carried along the sea floor from
continental margins via submarine
canyons which act as conduits for
turbidity current transport. Unlike
pelagic sediment snowfall which
provides a uniform cover, the turbidites
flowing on the bottom surface fill from
the seafloor upward to eventually cover
the abyssal hills. The original irregular
surface of a volcanic province remains
under the turbidite fill. These plains,
which may slope less than 1:8000, are
found adjacent to land masses –
extending from the continental rise to
the abyssal hills. On prominences that
rise above the plain, only sediments settling in the water column (pelagic deep-sea sediments) occur. On the surface of the
plain, the pelagic sediments are interbedded with a dominating sequence of sands, silts, and clays of terrigenous origin
that can be identified as turbidites by displaced benthic fauna and sediment patterns characteristic of turbidites.
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Deep Sea Trenches and Continental Rises
A
subduction zone is where two lithosphere plates converge and one slab plunges into the mantle. Because of the drag,
a trench is formed. Deep-sea trenches are long, narrow depressions in the ocean floor with depths greater than 6000
meters and they can reach 11,000 meters in depth. Trenches are found adjacent to land areas and associated with island
arcs worldwide. They are more numerous
in the Pacific Ocean. The trench is usually
asymmetric, with the steep side toward
the adjacent land mass. Where a trench
occurs off continental margins, the
turbidites from the slope are trapped,
forming a hadal plain on the floor of the
trench.
At the junction of continental and oceanic
crust where there is no trench, sediments
from the continental slope accumulate at
the base of the slope to form a continental
rise. The rises generally are adjacent to abyssal plains.
Volcanic Islands, Seamounts, Guyots, Atolls
V
olcanos reaching the ocean surface form volcanic islands. Subsidence of a volcanic island with growth of coral
keeping pace as it subsides will result in the formation of an atoll. Drowning of ancient volcanic islands by isostatic
adjustment is shown by the coral atoll deposits drilled in the Pacific. More than 1400 meters of shallow water carbonates - deposited in less than 100 meters water depth -- have been recovered from Bikini Atoll.
Guyots and seamounts are geomorphic forms developed from submarine volcanoes. They are isolated, but do lie in chains
or provinces of volcanic activity. They are found in all oceans, but more have been recorded in the Pacific Ocean. The
distribution that has been mapped may represent a small percent of the total number since they are only noted where
crossed during bathymetric profiling. The seamount is a relatively isolated elevation of the seafloor of more than 1000
meters height, with a small rounded top – a volcano that did not reach the sea surface. Guyots are drowned volcanic
islands that did not become coral atolls. They were planed flat by wave action when at shallow depths, after which
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subsidence occurred so that they are like seamounts but with flattened tops that lie more than 200 meters below the
surface. Although some coral rubble may be found on guyots, they are abrasional platforms that have subsided as a result
of isostatic adjustment, with some contributing effect from sea level change.
Continental Shelf and Slope
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he continental shelf (extending
from the shoreline to the shelf
break) is part of the continental crust
and water depths over the shelf are a
function of sea level and isostatic
change. The continental slope lies over
and is caused by the transition from
continental crust to oceanic crust.
Summary
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he most striking division of the earth's surface is into continental and oceanic areas underlain by oceanic and
continental crust of different density and thickness.
The major features of the sea floor are formed by tectonic processes of plate divergence, convergence, and transform
faulting. Volcanic activity is the dominant process on the surface of the sea floor and the final shaping is the emplacement
of volcanic features over the tectonic patterns.
The continental margins covered by present sea level are on continental crust. The morphology of these surfaces results
from tectonics, sediment deposition and a history of sea level changes.
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